* Ex-presidential guard Abdel Aziz seeks another 5-year term
* Seen in West as tough on al Qaeda
* Main challengers boycott vote, leaving weakened field (Adds observer comments, details)
By Joe Penney and Kissima Diagana
NOUAKCHOTT, June 21 (Reuters) - Voters trickled into polling centres in Mauritania on Saturday in an election where incumbent President Mohamed Ould Abdel Aziz was counting on a high turnout to see off an opposition boycott and boost his authority.
Abdel Aziz - an ally of Western powers in the fight against al Qaeda-linked Islamists in West Africa - is sure to win the poll in the desert nation straddling black and Arab Africa.
The bulk of the opposition boycotted last year's parliamentary elections - saying the organisers were biased and the process flawed - and talks to try to persuade them to take part in Saturday's vote broke down in April.
That left Abdel Aziz with no major rivals.
"Anyone can see that these (opposition) parties are void of any content and no longer even have a political role to play, reflecting the level of their leaders," Abdel Aziz, a former head of the presidential guard told reporters after voting.
Analysts believe the incumbent's main challenge will be to persuade enough voters to take part in the presidential vote and give him a strong mandate.
In the first hours of voting, turnout appeared low, even at major polling stations in the capital Nouakchott.
"There aren't too many people voting now because it's early in the morning and it's the weekend," Toinssi Cheikh, a trader, told Reuters after voting at the largely empty polling station at the Olympic Stadium in the city centre.
There was little increase in the flow of voters in Nouakchott by 3 pm, a Reuters witness said.
Voting is scheduled to end at 7 pm (1900 GMT) and ballot counting will begin immediately after.
An elections observer with the Arab Parliament, who asked not to be named as he was not authorised to speak to the press, said he had not heard of any reports of serious incidents during the vote.
"We have remarked that the operation is taking place in total calm and all the means have been put in place for the proper execution of the election," he said.
Mauritania's 2009 election saw 10 candidates vying for the presidency. In 2007, there were 19. But the opposition's boycott of this year's poll has gutted the field of potential challengers to Abdel Aziz, leaving the country's 1.3 million voters with just a handful of candidates to choose from.
The president's four remaining rivals are former government minister Boidel Ould Houmeid; Ibrahima Sarr, a challenger from the 2009 vote; Mint Moulaye Idriss, an administrator at Mauritania's national press agency and the only woman in the race; and anti-slavery campaigner Biram Ould Abeid.
Mauritania officially abolished slavery in 1980, but human rights experts say it remains one of the few countries in the world where the practice still exists.
"My candidacy, as I've always said, is a break with the past," Ould Abeid said after he voted. "Like Nelson Mandela in South Africa who brought hope to the blacks and like Barack Obama in the USA, I will be the first black slave descendant president in Mauritania."
The country has reserves of iron ore, copper and gold and is trying to boost investor interest in its oil and gas. However, it has long been plagued by political instability and military coups.
Abdel Aziz came to power in August 2008 when he ousted President Sidi Mohamed Ould Cheikh Abdellahi, the country's first democratically elected president, whose short stint as leader was undone by fighting within his own party.
He then won a 5-year term in a 2009 poll that was heavily criticised by the opposition, some of whom still do not recognise the legitimacy of his election.
Western nations soon re-engaged with Mauritania's military, which has taken a strong stand against Islamist groups in the country and neighbouring Mali.
Abdel Aziz sent his army, considered one of the most effective in West Africa, to carry out military strikes against Islamist bases in neighbouring Mali in 2010 and 2011. (Writing by Joe Bavier; Editing by Andrew Heavens and Raissa Kasolowsky)